The present invention relates generally to lead assemblies for connecting implantable medical devices with selected body tissue to be stimulated by such devices, and more particularly to techniques for providing a secure electrical and mechanical connection between wound elements, such as coil conductors, and mating parts such as electrodes, sensors and the like, employed within such lead assemblies.
Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing or for sensing electrical signals produced by the heart or for both pacing and sensing in which case a single lead serves as a bi-directional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart.
The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled or wound conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end with the electrode at the distal end.
When terminating a wound conductor to an associated electrical element such as a proximal end connector pin, a heart tissue stimulating electrode at the distal end of the lead, a blood oxygen sensor, or other such elements within the lead assembly, there is often no way to statistically ascertain the structural integrity of the termination. These joints must have a high degree of reliability for the implantable product to be acceptable for long term implants such as endocardial type pacing leads. In the past, the only way to verify the joint was to immobilize the mating part and pull on the wound conductor and this technique has been used as the chief test method. The major problem with this approach is that as the winding is pulled unequal tension is applied to the individual strains of the wound conductor. As increased tension is applied to the coil, often one strain breaks sooner than the others yielding erratic test results. The present invention provides an approach that overcomes this test method problem while at the same time providing a very reliable and secure connection between a wound element and a mating component.
Another problem associated with connections between wound elements and mating components in present day lead assemblies arises from the use of different alloys for the wound elements and mating components. Since dissimilar alloys have different melt temperatures and other thermal properties, such connections are difficult to weld. Moreover, as lead sizes decrease, problems of manufacturability arise. This is particularly true where crimping is employed to secure the wound component to a mating element. See, for example, U.S. Pat. No. 4,953,564, which discloses a cardiac pacing lead having an extendible fixation helix electrode that is mechanically and electrically connected to a rotatable conductor coil by squeezing the helix and coil together between a crimping sleeve and a crimping core. As the sizes of body implantable leads and their constituent parts become smaller, crimping becomes more difficult because the crimping tools cannot be made sufficiently small. Moreover, the same number of lead windings are not always subjected to the crimping action so that failure stress differs from lead to lead.
Some selective examples of the patented prior art will now be mentioned briefly. U.S. Pat. No. 5,569,883, to Walter et al., discloses laser-welding a wire coil to an intermediate ring or the like. U.S. Pat. No. 5,571,146, to Jones et al., discloses laser-welding dissimilar materials by means of an aperture within a lead. U.S. Pat. No. 5,385,578, to Bush et al., discloses laser-welding a wire coil to a sleeve.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.
The present invention relates to a method of joining a longitudinally extending wound element, or coiled wire strand, and a mating component, or post, of a body implantable lead assembly wherein the former has a longitudinally extending interior passage and an end portion adapted to be received by the latter. In one embodiment, the post is formed with an integral collar spaced from a terminal end thereof. The wound element is placed about the receiving portion and over the terminal end of the mating component and against the collar. The components are then joined by thermally fusing them together, preferably by means of a laser. If the collar and the wound element are fabricated of the same alloy, the thickness of the collar and the diameter of the coiled wire strand are designed to be substantially equal. If the components are fabricated of dissimilar alloys, then the thickness of the collar is relatively dimensioned with respect to the diameter of the strand in proportion to the relative square roots of thermal diffusivity of the alloy of the collar and of the alloy of the coiled wire strand. In another embodiment, a ring member is placed about, and in engagement with, the receiving portion of the mating component. Then the components are joined, by thermally fusing them together, preferably by targeting a laser beam directly on the ring member, without regard to whether the components are fabricated of the same alloy or of different alloys.
As already noted, a primary purpose of the invention is to improve a laser-weld between a winding and a connector and to achieve this result, the laser beam energy should be distributed equally between the wire and the connector. The common weld joint typically comprises a winding screwed onto a cylindrical connector. The very last wind (that is, the wire ends) sets against a shoulder. The shoulder and the last wind (the wire ends) are then welded together in an appropriate manner (see FIG. 1).
However, the problems which occur when this technique is attempted are at least twofold:
(1) the connector requires more laser energy to melt than does the wire; and
(2) the weld needs more melted metal to increase strength of the weld joint.
During welding, a laser beam melts both the connector and the wire (winding). The wire has less metal mass than does the connector. As such, the wire accumulates heat very quickly and the wire can melt easily. The wire melted metal spreads over the connector forming the weld spot. A lack of melted metal creates wire xe2x80x9cneck downxe2x80x9d and negative weld reinforcement, which reduces the strength of weld joint. The connector has much more metal mass, which means it draws the heat out of the weld region. This makes it difficult to melt the metal to fuse components together. Therefore, the connector requires more laser energy to melt than does the wire. To achieve a reliable weld, the beam energy must be specifically balanced between the connector and the wire. The proper beam targeting requires placement of the laser beam not equally on the joint such that more energy is on the shoulder side than on the wire. It is difficult for the line operator to target the laser beam on the joint properly.
A difference in material thermal properties magnifies the energy balance problem. For example, the platinum requires much more energy to melt then MP35N. If a joint consists of MP35N wire and a platinum connector it will need a greater misbalance of energy to melt the components equally. Proper beam targeting required to achieve a solid weld becomes more critical with dissimilar materials than with similar materials.
Consider now one instance of the invention when the joint is composed of components of dissimilar materials.
This initial embodiment includes a collar on the shoulder with a defined thickness. The collar thickness is related to the wire diameter, the wire and connector material thermal properties. According to the invention, the collar thickness can be determined by the equation presented below. The very last wind (wire ends) sets against the collar (see FIG. 2) and the collar and the winding (wire ends) are welded together (see FIG. 3).
A groove between the collar and the rest of the connector""s shoulder creates a heat flow barrier, accumulating the heat in the collar weld area. The accumulated heat in the collar equalizes heat condition in the wire and in the collar and does not require a specific energy balance. Weld joint can be created with energy distribution between the wire and connector as 50/50.
The heat flow balance between the wire and connector can be regulated by collar thickness. Higher thermal properties of a material require a thinner collar to compensate for heat flow misbalance. The material thermal property difference of both materials is incorporated in the formula by the ratio of square roots of thermal diffusivity quantity. Thus:
B={square root over (awire+L )}/ {square root over (aconnector+L )}xDxe2x80x83xe2x80x83(1)
where B=collar thickness,
D=wire diameter,
awire=wire material thermal diffusivity, and
aconnector=connector material thermal diffusivity.
In a special instance, if the connector and the wire are made from the same material, the collar thickness should be equal to the wire diameter.
In another instance of the invention, the proposed design comprises a connector, a winding and a ring member on the connector. The ring member sits on the connector against a shoulder. The very last wind (wire ends) sets against the ring member (see FIG. 4). The ring member thickness is related to the winding and connector material thermal properties and wire diameter. The size of the ring member is selected to provide melted metal to fill gaps, cavities between wire ends and create the weld positive reinforcement. The ring member, the winding and the connector are welded together (see FIG. 5).
A main portion of the laser beam energy goes to melt the ring member. The peripheral portion of the laser beam heats the shoulder and winding to warm them up, helping the components fuse together. Additional metal for the melted ring member covers the winding and the shoulder of the connector, fusing the components together and filling cavities and gaps between the wires (see FIG. 6).
This joint configuration, namely, a ring member between the winding and connector, does not require a specific energy balance. The laser beam is targeted right in the middle of the ring member. Further, the weld joint strength is increased due to the weld positive reinforcement.
A feature of this embodiment of the invention is that the connector, the wire and the ring member can be made from the same or different materials.
The shape of the ring member can be specifically profiled to control the volume of melted metal around the weld joint. The profiled ring member covers more cavities and gaps between the wire ends than the wire ends. FIGS. 6A and 7A illustrate two examples of the ring member profiles.
A primary feature, then, of the present invention is the provision of a significantly improved technique for providing a secure electrical and mechanical connection between wound elements, such as coil conductors, and mating parts such as electrodes, sensors and the like, employed within such lead assemblies.
Another feature of the present invention is the provision of such a technique employing a laser.
Still another feature of the present invention is the provision of such a technique which can achieve a satisfactory connection whether or not the alloys of which the components are fabricated are the same or dissimilar.
Yet another feature of the present invention is the provision of a technique of joining a longitudinally extending wound element and a mating component of a body implantable lead assembly, the wound element having a longitudinally extending interior passage and an end portion adapted to be received by a portion of the mating component, the receiving portion of the mating component being configured to receive the end portion of the wound element.
Still a further feature of the present invention is the provision of such a technique which includes providing the receiving portion of the mating component with an integral collar spaced from a terminal end thereof, then placing the end portion of the wound element about the receiving portion and over the terminal end of the mating component and against the collar, and then joining the collar and the end portion of the wound element.
Still another feature of the present invention is the provision of such a technique according to which the thickness of the collar is relatively dimensioned with respect to the diameter of the strand of the wound element in proportion to the relative thermal diffusivity of the alloy of the collar and of the alloy of the coiled wire strand.
Yet a further feature of the present invention is the provision of such a technique which includes placing the end portion of the wound element about the receiving portion of the mating component, placing a ring member about, and in engagement with, the receiving portion of the of the mating component and joining the ring member, the end portion of the wound element and the receiving portion of the mating component.
Still another feature of the present invention is the provision of such a technique which includes targeting a laser beam on the ring member and thermally fusing the ring member, the end portion of the wound element and the receiving portion of the mating component.
Yet another of the present invention is the provision of such a technique wherein the ring member has inner or outer peripheral surfaces profiled to control the volume of melted, then fused, metal forming the weld joint.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.